Atmospheric rotary sterilizers are used to sterilize containers of food products that are high in acid content, while pressure sterilizers are used for food products low in acid, which require more heating to achieve thorough sterilization of the food. Atmospheric sterilization is advantageous over pressure sterilization from an equipment standpoint because atmospheric pressure levels greatly simplify equipment design and operation. With pressure sterilizers, pressure levels as high 45 psi and more are used to sterilize food containers, which requires that transfer valves be sealed to maintain the high pressure levels within the sterilization vessels. Sealed transfer valves require greater power inputs to operate, which complicates associated drive systems. In fact, pressurized transfer valves can utilize up to 80% of the power requirements of a rotary sterilizer system. The high power inputs to transfer valves require complex gearing and precise valve positioning in order to rotate the transfer valves, which complicates design of the drive system machinery and significantly adds to its cost. Complex gearing also limits a sterilizers ability to handle different size containers due to the specific timing requirements of different sized containers.
A disadvantage of atmospheric rotary sterilizers is the upper temperature limits of these machines. Typically, an atmospheric rotary sterilizer is limited to process temperatures of 206 to 210 degrees Fahrenheit, depending on elevation. Pressure sterilizers, of course, can achieve significantly higher temperatures and, thus require shorter processing times.
A rotary sterilizer, either pressure of atmospheric, operates generally as follows. Filled containers enter a processing line by means of a positive feed device, which synchronizes the cans with the rest of the line. From the feed device, the containers are transferred to a rotary valve on the sterilizer vessel, which, in the case of a pressure sterilizer, is designed to prevent the escape of steam from the vessel shell. Inside the vessel shell, containers are positively conveyed by a spiral and reel mechanism. The reel rotates and pushes the containers along the spiral. The containers are then ejected from the reel to a rotary discharge valve and into the next sterilizing vessel or cooling vessel, as the case may be.
Within the sterilizer vessel, containers are heated by steam from a trough on the bottom of the vessel. Uniform distribution of steam is ensured by a manifold steam supply system feeding the trough along the entire length of the vessel. Bleeders are placed at frequent intervals along the top of the vessel and in the transfer valves of pressure sterilizers. Such continuous air removal ensures uniform temperature control, as required by safety regulations.
The rotor in each transfer valve is constructed with equally spaced sprockets on the valve's periphery. Each pocket is designed to hold one can and is mechanically timed with the station on the reel. At the discharge end, a star wheel mounted within the reel gently ejects the containers into the discharge valve for removal from the vessel.
Atmospheric rotary sterilizers typically feed containers directly onto the sterilizer's reel within its vessel. The containers are fed by a free roller feed or a feed chute through an inlet opening in the vessel shell. Due to low steam pressures within the vessel and also due to the size of the opening relative to the size of the cans, a minimal amount of steam and air escapes through the inlet opening.
The present invention is designed to provide a low pressure sterilizer and transfer valve mechanism that achieves advantages of the both atmospheric and pressure sterilizers.